A microtomograph is a device that produces a high spatial resolution image of a specimen such as biological tissue, an electronic component, or a geological sample. One type of microtomograph in use today is the x-ray microtomograph. This device typically includes an electron gun that produces a high voltage electron beam, which is directed to strike a solid or thin foil target formed from a metal having a relatively high atomic-number. The impact of these high energy electrons on atoms within the target foil causes an x-ray beam to be emitted by the target. The microtomograph is configured so that the x-ray beam is directed toward the specimen being imaged. An x-ray sensitive detector is located beyond the specimen to monitor the transmission of x-rays through the specimen. The signal produced by the detector in response to the x-rays is then processed to generate an image of the specimen. Currently, most microtomographs are capable of producing specimen images at a spatial resolution on the order of 50-200 microns.
In the past few years, it has been recognized x-ray microtomographs can be constructed from scanning electron microscopes (SEMs). An SEM has an electron gun that can be configured to produce an electron beam that is directed toward a foil target, causing an x-ray beam to be emitted for use in imaging a specimen. However, a disadvantage of some of these microtomographic systems constructed from SEMs is that owing to the nature of the construction, the specimen and detector can only be positioned in a few locations relative to the x-ray source. This restriction limits the flexibility in the size of specimens that may be accommodated and the spatial resolution of the resultant images.
While current x-ray imaging devices, including x-ray microtomographs constructed out of converted SEMs, have proved useful devices for generating relatively high-resolution images, they are not without limitations. One limitation of most x-ray microtomographs is that they can only produce two-dimensional images. The only way that three-dimensional images can be created with such systems is by making a series of two-dimensional slice images and then interpolating the specimen structure between the slice lines.
Attempts have been made to design microtomographs capable of directly producing three-dimensional images. Some of these devices are designed to simultaneously expose the specimen being viewed to two or more x-ray beams. A disadvantage of these devices is that they require multiple x-ray beam generators and a complementary number of x-ray detectors. The expense as well as the practicality of assembling the large number of components these microtomographs require has limited their potential for providing three-dimensional high resolution images.
Another disadvantage of currently available microtomographs is that they are unable to generate images providing quantitative information about the composition, or phase fractions, of the material comprising the specimen being viewed. While many microtomographs are capable of generating images that qualitatively depict the existence of two or more phases of a specimen, it has been difficult to use a microtomograph to generate a quantitative value indicating, for example, the ratio of hard tissue to soft tissue in a biological tissue specimen. This ability to quantify the phase fractions of the tissue is important for the study of, for example, bone structure and diseases such as osteoporosis. Although attempts have been made to construct microtomographs that provide quantitative data about the specimen being imaged, the current processes only provide data that serve as an estimate of the phase fraction of the material, but do not provide data that precisely quantify the phase fraction of the material being imaged.
The inability of current microtomographs to provide such precise phase fraction related quantitative information about the specimen being imaged, as well as the lack of simple, cost effective microtomographs capable of directly making three-dimensional images, has thus limited the use of these instruments.